Neither party requested a jury trial and the case was tried by Judge Lewis deciding both questions of law and fact. Where the trial judge sits as the trier of the facts, his findings of fact are conclusive on appeal when supported by competent evidence. This is true even though there may be evidence in the record to the contrary which could sustain findings to the contrary. Whitaker v. Earnhardt, 289 N.C. 260, 221 S.E. 2d 316 (1976); Associates, Inc. v. Myerly and Equipment Co. v. Myerly, 29 N.C. App. 85, 223 S.E. 2d 545, dis. rev. denied and appeal dismissed, 290 N.C. 94, 225 S.E. 2d 323 (1976). The trial judge is both judge and jury, and he has the duty to pass upon the credibility of the witnesses who testify. He decides what weight shall be given to the testimony and the reasonable inferences to be drawn therefrom. The appellate court cannot substitute itself for the trial judge in this task. Knutton v. Cofield, 273 N.C. 355, 160 S.E. 2d 29 (1968). Judge Lewis had the duty to find facts, state separately his conclusions of law and enter judgment. N.C. Gen. Stat. 1A-1, Rule 52(a)(1); Coggins v. City of Asheville, 278 N.C. 428, 180 S.E. 2d 149 (1971). It is incumbent on this Court to review the evidence to determine if it supports the findings of fact presented by assignments of error on appeal. Whitaker v. Earnhardt, supra. We discuss the required findings of fact separately.
*276 [1,2] In plaintiffs first claim, it alleges extra expense for storage of materials in a dry warehouse off the project site, in order to comply with defendant’s work schedule. The following evidence sustains this finding:
There was no dry place on the site where the material could be stored; therefore, we asked permission to store the material off the site. . . .
. . . The agreement with Mr. DuBose, Assistant Vice President of Nello Teer, was that we would be allowed to store the material in a bonded warehouse off the site since there was no facility at the site to store it.
. . . Irvin Chatham determined (job superintendent for Nello Teer) that it would not be practical to store this material within the building. . . .
. . . Our total cost for storing the material and glass at the warehouse was $8,520.98.
Teer was a party to this agreement through its vice president, DuBose. It had knowledge of the off-site storage before it was ever begun. Defendant relies on the provision in the contract requiring notice of any claim against contractor (Teer) to be given in writing within seventy-two hours after the cause of the claim occurred, citing Construction Co. v. Highway Comm., 28 N.C. App. 593, 222 S.E. 2d 452 (1976). It is true that in Construction Co. the Court upheld a notification clause of the contract in litigation. However, in that case there was no evidence of any additional agreement between the parties covering alleged extra work. To the contrary, officials of the Commission told plaintiff that it would be a “waste of time” to discuss it. Here, Teer waived any rights it had under the subcontract to written notice by its agreement through DuBose with plaintiff for the off-site storage.
Plaintiff’s third claim for relief is admitted by defendant’s answer, except as to the amount of plaintiff’s damages. This claim involved changes in the plans and specifications for the corner ridges. Plaintiff was ordered to make these changes by Teer in a *277letter to plaintiff dated 19 March 1973. Plaintiff’s evidence sustained a finding that the required changes resulted in additional costs to plaintiff of $10,886.32.
 Plaintiff’s fourth claim involves defendant’s installing the curtain wall support steel with improper tolerance, requiring plaintiff to realign and modify the curtain wall grid system at an alleged cost of $57,249.40. Judge Lewis found that all the parties knew of the required tolerances in the construction of the curtain wall, and that the installation of the structural and support steel was probably incompatible with the curtain wall, and that this probably resulted from the project design. These findings are amply supported in the record by the evidence of DuBose and Smith, who testified in detail as to the requirements of the support steel in relation to the curtain wall.
Judge Lewis further found this claim was controlled by Article 19 of the subcontract which stated the subcontractor (plaintiff) could not recover for extra work unless the contractor gave a written order for the work. The record sustains this finding as there is no evidence of any waiver by defendant, Teer, of this requirement. There is no evidence that Teer issued such written order with respect to this work by plaintiff, or that plaintiff requested a written order from Teer.
The remaining findings are not the subject of assignments of error nor argued in the briefs of either party. The findings of fact support the separate conclusions of law stated by Judge Lewis allowing plaintiff to recover on its first and third claims, in the total amount of $19,751.45 and denying plaintiff’s fourth claim for relief. Knutton v. Cofield, supra.
This complicated case was carefully tried by the able and experienced trial judge, and his judgment is
Judges Vaughn and Erwin concur.